1. Field of the Invention
The present invention relates to a data storage devices such as a hard disk drive, and relates in particular to controlling the position of the head of the data storage device when reading or writing data.
2. Background Art
A hard disk drive is one of the most popular external storage devices available for computers. As is well known, the surface of a magnetic disk that serves as a storage medium for a hard disk drive is divided into tracks that have the appearance of the rings of a tree, and each track is further radially divided into sectors that serve as the minimum data recording units. As a result, the linear recording density increases the nearer a track is to the outer circumference of the magnetic disk. Therefore, for the zoned bit recording method that is currently the primary data recording method for a magnetic disk, all the tracks are grouped into a number of zones, and a constant linear recording density is defined for each zone. Then, since the length of a sector is generally 512 bytes, the number of sectors increases the nearer a track is to the outer circumference of the magnetic disk.
A hard disk drive has magnetic heads for reading data from a magnetic disk, or for writing data thereto. These magnetic heads are mounted on an actuator mechanism that is driven by a VCM (Voice Coil Motor).
When a magnetic head reads or writes data, the actuator mechanism is driven and moves and positions the magnetic head above a predetermined track. For the moving and the positioning of the magnetic head at a predetermined location, servo information recorded on the magnetic disk is employed.
The positioning control provided for the magnetic head is exercised by switching among three modes, a seek mode, a settling mode and a following mode, in accordance with the distance between the current position of the magnetic head and the target position to which the magnetic head should be moved.
For example, when a seek request is issued to move the magnetic head to a target position while the head is located at X, as is shown in FIG. 2, the positioning of the magnetic head is performed by setting in order the seek mode, the settling mode and the following mode. It should be noted that the vertical axial direction in FIG. 2 represents distance and that the horizontal axial direction represents time, and that the movement of the magnetic head, which begins with the seek mode, is called a long seek. When a seek request for moving the magnetic head is issued while the head is located at Y, which is nearer the target position than is X, positioning control is exercised by setting in order the settling mode and the following mode. The moving of the magnetic head beginning with the settling mode is called a short seek. In FIG. 2, the seek mode is selected when the distance between the current position of the magnetic head and the target position exceeds distance B, the settling mode is selected when the distance between the two points is greater than distance A and less than distance B, and the following mode is selected when the distance between the two points is equal to or less than distance A.
The seek mode is a control mode wherein the velocity of a head is calculated based on the distance to a target position so as to follow up with the movement of the head. The settling mode is the feedback control based on the distance to the target position and the velocity. The following mode is the control obtained by employing an integrator for the settling mode. As is described above, since the control method differs in these modes, the feedback parameter also differs.
Since the feedback parameter is suddenly changed when the control modes are switched, it has been pointed out that during this switching process the action of a position control current becomes unnatural. This problem is called a “continuity” problem.
FIG. 12 is a graph illustrating the “continuity” problem. The graph shows the velocity and the position of a head and a current (DAC out) supplied to an actuator for moving the head during a seek process performed for a hard disk drive. In FIG. 12, when the seek mode is changed to the settling mode, the current DAC out is changed greatly. The great change in the current DAC out causes acoustic noise, and also increases the seek time.
It is further understood that a contradiction occurs when the feedback parameter in the settling mode is employed both for a long seek, performed in the seek, settling and following mode order, and for a short seek, which begins with the settling mode. This is because the initial velocity that is set when the settling mode is entered differs between a long seek and a short seek. That is, during a long seek the head enters the settling mode at a specific initial velocity, while in the short seek, the initial velocity is zero.
Naturally, when the head enters at an initial velocity, the feedback gain for a controller will be smaller than when the initial velocity of the head is 0. Therefore, when the feedback parameter in the settling mode is adjusted to an optimal value during a short seek (when the initial velocity is 0), the head will overshoot during a long seek. And when the parameter in the settling mode is adjusted for a long seek, in a short seek the gain will not be sufficient and the head will undershoot a track. This is a contradiction. In order to resolve this contradiction, according to one method that is employed two settling parameters are used during a long seek and a short seek; however, this method has not been successful.